Myosin



Myosin is one of three major classes of molecular motor proteins: myosin, dynein, and kinesin. As the most abundant of these proteins myosin plays a structural and enzymatic role in muscle contraction and intracellular motility. Myosin was first discovered in muscle in the 19th century. Myosin is a superfamily of proteins which bind actin, hydrolyze ATP and transduce force. Thus most are located in muscle cells. Composed of head, neck and tail domains. Head domain binds the actin and moves along it. The neck is a linker and binds the light chains which have a regulatory function. The tail interacts with cargo molecules (CBD)m. There are 18 classes of myosin. Myosin II (MII) is best studied and contains 2 heavy chains (HC) which constitute the head and tail domains and 4 light chains (LC) which are referred to as the essential LC (ELC) and the regulatory LC (RLC). The images at the left and at the right correspond to one representative Myosin, i.e. the crystal structure of from Myosin Gallus gallus (2mys).

Crystallization and X-ray diffraction
Myosin is found in abundance, therefore it can be prepared in gram quantities. For nearly 30 years the myosin head was resistant to crystallization, yet by 1993 researchers discovered a mechanism to obtain x-ray quality crystals. The process modified the protein by reductive methylation. X-ray data was used to determine the tertiary structure of the protein.

Structure
Myosin has a molecular size of approximately 520 kilodaltons with a total of six subunits. It has two 220 kD heavy chains which make the majority of the overall structure and two pairs of light chains which vary in size. The molecule is asymmetric, having a long tail and two globular heads. Each heavy chains composes the bulk of one of the globular heads. Subfragment-1(S1) also termed the myosin head consists of ATP, actin, and two light chain binding sites. Each globular head has a heavy chain and two light chains for a combined molecular size of about 130 kD.

The myosin head is assymetrical with a length of 165 Angstroms and 65 Angstroms in width, with a total thickness of about 40 Angstroms. About 48% of the amino acid residues in the myosin head are dominated by α helices. At the carboxyl terminus one long α helix of about 85 Angstroms extends in a left-handed coil. This particular helix forms the light chain binding region of the globular domain The amino terminus of each heavy chain has a large globular domain containing the site of ATP hydrolysis.



Function
Molecules of myosin aggregate in muscle cells to form thick filaments. The rodlike structure of these thick filaments act as the core in the muscle contractile unit. The aggregation of several hundred myosin forms a bipolar structure which stacks in regular arrays. Muscles consist of another protein called actin. Actin forms the thin filament in muscle fibers. Myosin and actin interact through weak bonds. Without ATP bound, the myosin head binds tightly to actin. With ATP bound, myosin releases the actin subunit and interacts with another subunit further down the thin filament. This process continues in cycle, producing movement. Interaction of myosin and actin is regulated by two other proteins, tropomyosin and troponin.

The cycle of myosin-actin interaction is outlined as follows:

1. ATP binds to myosin and a binding site opens on myosin head to disrupt the actin-myosin interaction, actin is released. ATP is hydrolyzed

2. a conformational change moving the protein to a "high-energy" state causes the myosin head to change orientation moving it to bind with the actin subunit closer the a region called the Z disk than the previous actin subunit

3. the binding site is closed, strengthening the myosin-actin binding

4. a power stroke quickly follows and the myosin head undergoes an additional conformational change bringing it back to the resting state in which it began

Click the link to access DNAtube video "A Moving Myosin Motor Protein" http://www.dnatube.com/video/389/A-Moving-Myosin-Motor-Protein-myosin-actin-interaction

3D Structures of Myosin
Update June 2011

Myosin I
1lkx – DdMI HC - Dictyostelium discoideum

2drk, 2drm – MI HC + 10-mer peptide – Acanthamoeba castellanii

Myosin II
1o1g, 1o1a, 1o1b, 1o1c, 1o1d, 1o1e, 1o1f, 1o18, 1o19, 1mvw, 1m8q – cMII RLC+ELC+HC+actin – tomography - chicken

3lla – cMII HC alpha-kinase domain+AMPPCP

3lmh – cMII HC alpha-kinase domain+ADP

3lmi – cMII HC alpha-kinase domain (mutant) +ATP

2fxm, 2fxo – hMII HC S2 fragment - human

3i5f – LpMII RLC+HC+ADP+Mg – Loligo pealei

3i5g, 3i5h – LpMII RLC+HC+ELC

3i5i – LpMII RLC+HC+ELC+SO4

2jhr – DdMII HC+ADP-VO4+pentabromopseudilin

2xo8 - DdMII HC motor domain +pseudilin derivative

3bz7, 3bz9, 1iv3, 3bz8 - DdMII HC+blebbistatin

3mjx - DdMII HC+blebbistatin + ADP-VO4

3mnq - DdMII HC motor domain + ADP-VO4 + reservatrol

3bas, 3bat, 1fmv - DdMII HC

1fmw – DdMII HC+ATP

1jwy, 1jx2 – DdMII HC+dynamin-1

1d0x, 1d0y, 1d0z, 1d1a, 1d1b, 1d1c – DdMII HC (mutant)+BeF3 derivative

1g8x – DdMII+actinin 3

2jj9, 2x9h - DdMII HC+ADP-VO4

3mkd - DdMII HC motor domain (mutant) + ADP-VO4

1lvk, 1mma, 1mmg, 1mmn – DdMII HC (mutant)+Mg+nucleotide

1myh, 1myk, 1myl, 3myh, 3myl - DdMII HC (mutant)

1mne - DdMII HC (mutant)+Mg+pyrophosphate

1vom - DdMII HC (truncated)+Mg+ADP-VO4

1mmd, 1w9i, 1w9k - DdMII HC (mutant)+Mg+ADP+BeF3

1mnd, 1w9j, 1w9l - DdMII HC (mutant)+Mg+ADP+AlF4

2aka - DdMII HC+dynamin-1

1n2d – ScMII LC+IQ2 IQ3 motifs from Myo2p

1m45 - ScMII LC+IQ2 motif from Myo2p

1m46 - ScMII LC+IQ4 motif from Myo2p

2bl0 – MII RLC +RHC – Physarum polycephalum

Myosin III
2btt – ScMIII SH3 domain – NMR

1ruw, 1va7 – yMIII SH3 domain

Myosin IV
3mmi – yMIV globular tail

Myosin V
1w7i – cMV HC+LC+Mg+ADP

1w7j - cMV HC+LC+BEFX+ADP

1w8j – cMV HC

1oe9 – cMV HC+LC

1br2 – cMV HC+Mg+ADP+AlF4

2fcd – ScMV LC N-terminal – Saccharomices cerevisiae – NMR

2fce – ScMV LC C-terminal – NMR

2f6h – ScMV CBD

1yp5 – yMV SH3 domain

2ix7 – mMV+apo-calmodulin - mouse

Myosin VI
2kia – mMVI CBD – mouse – NMR

3h8d - mMVI CBD+Dab2 peptide

3gn4, 2vb6 - pMVI neck+calmodulin – pig

2vas, 3l9i - pMVI neck (mutant)+calmodulin

2v26 - pMVI neck+Mg+ADP-VO4

2bkh, 2bki – pMVI HC+calmodulin

2x51 - pMVI d insert1 + calmodulin

Myosin VII
2i0n - DdMVII SH3 domain - NMR

3pvl - mMVII SH3 domain+hUsher syndrome type 1G protein

Myosin X
3pzd - hMX myth4-ferm tandem + netrin receptor DCC

Flight muscle myosin
1i84, 2w4a, 2w4g, 2w4h - cRLC+cELC+cHC – cryoEM

2mys - cRLC+cELC+cHC - papain digested

1lkm – cHC alpha-kinase domain+AMP

2dfs – cHC+calmodulin

1br4 – cELC+cHC+Mg+ADP+BeF3

1br1 – cELC+Mg+ADP+AlF4

2xrf – hLC

3jtd, 2w4t, 2w4v, 2w4w, 1scm – AiRLC+AiELC+AiHC - Argopecten irradians

1b7t - AiRLC+AiELC+AiHC papain digested

3jvt - sRLC+sELC+sHC+Ca – Scallop

2ec6, 2os8, 2otg, 1s5g, 1sr6, 1qvi, 1kk7, 1dfk, 3pn7 - sRLC+sELC+sHC

1kqm - sRLC+sELC+sHC+AMPPNP

1kwo - sRLC+sELC+sHC+ATPgS-PDM

1l2o - sRLC+sELC+sHC+ADP-PDM

1kk8 - sRLC+sELC+sHC+ADP-BEFX

1dfl - sRLC+sELC+sHC+ADP-VO4+Mg

1wdc - sRLC+sELC+sHC - digested

3dtp - RLC+HC+ELC – tarantula – Cryo EM